With the spectacular diversification and expanding volume of information in recent years, various new recording processes and materials have been developed and put to practical use. Of these, the heat-sensitive recording process has many advantages: (1) desired prints can be obtained simply by heating heat-sensitive materials, with no need for cumbersome development process; (2) heat-sensitive recording materials can be manufactured and stored more easily and at less costs than any other type of recording material; and (3) when paper, which is an inexpensive material, is used as the substrate, as is often the case, the resulting heat-sensitive recording materials have a favorable feel like plain paper.
Heat-sensitive recording materials commonly used for this purpose are composed of a suitable substrate, such as paper, synthetic paper and synthetic resin film, and a heat-sensitive, color-developing layer coated thereupon which contains a colorless or faint-color leuco dye and an organic acid that is capable of reacting with said leuco dye to develop color upon heating.
More recently there has been a great demand in this field for improved heat-sensitive recording materials adapted for high-speed recording which will give prints of sufficient density with shorter heating time of, and lower voltage applied to, thermal heads.
In order to meet this requirement, Japanese patent application Laid-open No. 39,139 (1978) teaches a heat-sensitive recording material containing, in its color-developing layer, at least one compound melting at a temperature in the range from 60.degree. C. to 200.degree. C. and selected from alkylbiphenols and substituted biphenylalkanes. Japanese patent application Laid-open No. 48,751 (1978) also proposes a heat-sensitive recording material containing a fusible substance which melts at a temperature in the range from 60.degree. C. to 200.degree. C.
These prior arts, however, are still unsatisfactory in terms of thermal response and print quality in high-speed recording.
The primary object of this invention is to provide a heat-sensitive recording material which shows quick thermal response and hence is capable of giving sharp and clear prints of high density even in high-speed recording. A further object of this invention is to provide a heat-sensitive recording material which contains, in its color-developing layer comprising a leuco dye and an organic acid capable of reacting with said leuco dye to develop color upon heating, a specific hydrocarbon compound that can act as a color development accelerator. A still further object of this invention is to provide a heat-sensitive recording material having a heat-sensitive, color-developing layer which contains a leuco dye and an organic acid that reacts, upon heating, with said leuco dye to develop color, said heat-sensitive, color-developing layer also containing at least one hydrocarbon compound melting at a temperature in the range from 50.degree. C. to 200.degree. C. and selected from the group consisting of benzylbiphenyls, alkyl-substituted benzylbiphenyls, hydrogenated benzylbiphenyls, hydrogenated alkyl-substituted benzylbiphenyls, hydrogenated terphenyls and hydrogenated alkyl-substituted terphenyls.